The heat transmitting tube for the boiler is manufactured so as to efficiently contact with combustion gas of fossil fuel or high temperature process gas. For this end, the heat transmitting tube frequently contacts with various corrosive impurities contained in the gas such as sulfur oxide (SOx) and nitrogen oxide (NOx), or vanadium compounds (V.sub.2 O.sub.5, NaVO.sub.3, Na.sub.2 O.V.sub.2 O.sub.5 and the like) and sulfur compounds (Na.sub.2 SO.sub.4, K.sub.2 SO.sub.4 and the like) included as a combustion ash content, and so on and hence is liable to be chemically damaged. Particularly, the heat transmitting tube for the boiler burning a heavy oil fuel containing the vanadium compound and the sulfur compound is considerably worn out by accelerated oxidation corrosion resulting from the vanadium compound and sulfurization corrosion of the sulfur compound. These corrosion damages are called gas-side corrosion because they are created at the outer surface of the heat transmitting tube or a position contacting the combustion gas.
As a method of preventing gas-side corrosion, there has hitherto been proposed a method of forming protective coatings on the surface of the heat transmitting tube as mentioned below.
(1) In JP-A-61-41756 is disclosed a technique in which Ni--Cr alloy or self fluxing alloy is sprayed onto the surface of the heat transmitting tube for a fluidized bed type boiler burning coke and then fused by heating to impart heat resistance and abrasion resistance to the heat transmitting tube.
(2) In JP-A-60-142103 is disclosed a technique a self fluxing alloy coating is formed on the surface of the heat transmitting tube for a boiler covering waste heat in a dry type fire extinguishing device and fused by heating and further subjected to a solid solution treatment or an annealing treatment to prevent erosion.
The above two techniques are effective in boilers used under an environment in which the abrasion rate is larger than the corrosion rate.
(3) In JP-A-2-185961 is disclosed a technique in which Al is coated onto the surface of the heat transmitting tube for the boiler by spraying and a self fluxing alloy sprayed coating containing Al is formed thereon and then fused by heating to impart corrosion resistance to the heat transmitting tube.
(4) In JP-B-7-6977 and JP-B-7-18529 is disclosed the formation of a sprayed coating on the heat transmitting tube for the boiler.
As the corrosion damage created in the boiler, there is a water-side corrosion observed in an inner wall face of the heat transmitting tube or a surface passing a boiler water or an overheated steam therethrough in addition to the above gas-side corrosion. In general, the boiler water is usually adjusted to an alkalinity for controlling the above water-side corrosion. Therefore, as the operation of the boiler is continued over a long time period, an alkali component contained in the boiler water locally concentrates at the inner wall face of the heat transmitting tube and hence the tube material is corroded to produce an iron oxide. And also, compounds of Si, Ca, Mg, P, Cu and the like slightly contained in the boiler water precipitate on the inner wall face of the tube. As a result, obstruction of heat transmission is caused but also a phenomenon such as local overheating or the like is caused, and the heat transmitting tube is sometimes broken by these causes.
These phenomena are created in a portion of an evaporation tube producing steam by boiling of the boiler water. This portion is a neighborhood of a fuel combustion region having a greatest heat loading in view of the boiler structure. As seen from the above explanation, the position generating the corrosion damage due to the boiler water is restricted to a side that the heat transmitting tube for the boiler is always subjected to heat loading, while there is no problem in an opposite side not being exposed to the combustion gas.
As mentioned above, the conventional heat transmitting tube for the boiler, particularly the evaporation tube portion has the following problems.
(1) Since the heat loading in the inner wall face of the evaporation tube is high, alkali component in the boiler water is concentrated to cause thickness reduction through corrosion of the inner wall face of the tube. PA1 (2) At a portion violently evaporating water under a high heat loading, components dissolved in the boiler water such as Ca, Mg, Si, Fe, P, Cu and the like are precipitated to ununiformly adhere and deposit onto the inner wall face of the tube. PA1 (3) The substance adhered onto the inner wall face of the tube is poor in the thermal conductivity, so that temperature of the inner wall face in the tube facing the combustion gas (heat transmitting face) abnormally rises and hence the formation of oxide scale is promoted or the breakage of the tube is induced. PA1 (4) When a substance precipitated onto the inner wall face of the tube or deposition grows large, it is apt to be locally peeled off therefrom. As a result, the boiling of water becomes violent in the peeled portion, which promotes the phenomena of the above items (1), (2). Therefore, corrosion through alkali component locally progresses to wear out the tube wall. PA1 (5) When the peeling of the deposition is at a half-finished state or when a crack is caused in the deposition, the boiler water penetrated is immediately rendered into steam. Since steam is very low in the thermal conductivity as compared with water, the inner wall face of the tube is locally over-heated and hence cracks are created in the heat transmitting tube itself to sometimes bring about breakage.
It is, therefore, a main object of the invention to propose a technique of controlling the adhesion of the deposition onto the inner wall face of the heat transmitting tube for the boiler.
It is another object of the invention to propose a technique of mitigating heat loading in the heat transmitting tube for the boiler to prevent corrosion in the inner wall of the tube.
It is another object of the invention to propose a surface coating material of a heat transmitting tube for the boiler effective for mitigating corrosion through alkali component in the boiler water and preventing local over-heating.
It is a still further object of the invention to propose a technique of forming a sprayed coating for improving a service life of a heat transmitting tube for the boiler.
It is another object of the invention to propose a method of forming a sprayed coating effective for mitigating heat loading in an outer surface of a heat transmitting tube for the boiler and a method of manufacturing the heat transmitting tube for the boiler having an excellent effect of controlling the adhesion of the deposition.